1. Field of the Invention
The invention relates to a shift device for actuating at least one gear mechanism on a bicycle in accordance with the preamble of claim 1.
2. Description of Related Art
A shift device for controlling bicycle gear mechanisms such as derailleur shift mechanisms or multispeed hubs has, been disclosed by German Utility Model DE 90 15 515 U1, which has a housing comprising a housing plate and a housing cover which are held together by a central fastener, furthermore has a cable pulley, which is wound onto the Bowden cable leading to the bicycle gear mechanism, and finally has a shift lever which is held in a central position by a double-acting torsion spring and, when actuated in either of two directions of rotation, in each case shifts to the next gear, there being arranged on the lever two detent pawls which act in opposite directions, as regards the direction of rotation, on a ratchet wheel which is connected in a rotationally fixed manner to the cable pulley, one of the two detent pawls rotating the ratchet wheel by its engagement with the latter, while the other detent pawl is simultaneously disengaged.
Such an arrangement of the detent pawls, which act inwards in opposite directions on a ratchet wheel, is a traditional solution which is frequently employed for gear changing in bicycle hubs and shift devices but is too complex for use in a shift device and, especially, is too bulky in terms of its space requirements. The solution proposed in accordance with the present invention is limited to a single detent pawl which controls a bicycle gear mechanism in conjunction with the tensile force of a cable, the detent pawl being able to engage in detent toothing in a housing when the rotary motion is interrupted counter to the direction of the cable tension. Gear changing in the opposite direction of rotation, i.e. when the shift device ought to yield to the cable tension of the cable, is achieved by simply lifting out the detent pawl, the cable tension rotating a rotary element with a spool back until the detent pawl can reengage in the detent toothing of the housing. As can be seen, gear changing is performed against the tension in the cable in one direction of rotation, the rotary element with the spool being twisted in a process involving the expenditure of force, whereas, in the opposite direction of rotation, it is merely a matter of exercising control, the detent pawl merely being lifted out of its detent toothing in the housing by means of a release projection on a grip element. The cable tension in the cable preloaded by the bicycle gear mechanism corresponds to a preload on the rotary element in the direction of cable release, with the result that the detent pawl is pulled abruptly from the release position in the detent toothing to the next tooth of the detent toothing, this corresponding to one gear ratio in the gear shift. The gear change can be concluded before the shift operation by turning the grip element is ended. The shifting of the bicycle gear mechanism in the direction of winding up thus corresponds to a forcible adjustment of the bicycle gear mechanism, e.g. a derailer shift, which is preferably used to swivel a bicycle chain from a smaller gear ring to a larger gear ring since it is more difficult to transfer the chain in this direction than in the opposite direction, namely when transferring the chain from a larger gear ring to a smaller gear ring.
However, the grip element can also be used to preload the rotary element in the direction of cable release, this being accomplished by virtue of the fact that, when the release projection is moved by rotating the grip element in the direction of the detent pawl, the spring element is already preloaded and, when the detent pawl disengages, propels the latter into the next notch of the detent toothing by means of the sum of the preloading force of the spring element and the tension in the cable, thereby turning the rotary element further by the rotational angle of one gear.
It is thus the object of the invention to provide a shift device which, for reasons of cost, comprises a minimum number of components, in particular a single detent pawl, and is nevertheless capable of meeting all the requirements on a twist-grip shift device controlled by detent pawl.
The way in which this object is achieved is described in the defining part of the main claim. Refinements will be found in the subclaims.